The present invention relates to an improved method of and apparatus for bending glass or other sheet material heated to a formable state. More particularly, the present invention relates to a method and apparatus for bladder-molding glass.
A conventional glass bending apparatus typically includes two rigid molds--a male mold and female mold--which are brought together to conform a heated sheet of glass positioned therebetween to the shape of the two molds during the press cycle of the bending process. Some bending methods may include a slump bending step, which is initiated before the press cycle begins. In such a step, the heated glass sheet may be held in a holder that includes a generally concave, upper support surface, formed by a formable material such as a stainless steel mesh sheet. Since the sheet of glass is heated to a temperature in the range of 1000 degrees Fahrenheit to 1250 degrees Fahrenheit and is, therefore, in a partially molten state, the glass sheet partially conforms to the concave shape of the mesh sheet. Thereafter, as the female mold is moved towards the male mold, the female mold presses against the edge portions of the heated sheet of glass and the male mold presses on the center of the heated sheet of glass to start the press cycle of the bending process. As the two molds are pressed together, the bearing contact and pressure exerted by the molds on the edge portions and in the center causes the molds to further bend the sheet of glass. As the sheet bends, the bearing contact and pressure from the female mold expands from the edge portions of the sheet inward until it meets with the bearing pressure from the male mold, somewhere between the center and the perimeter of the sheet. Consequently, as the bearing pressure expands inwardly from the perimeter of the glass sheet, buckles form at the periphery of the plate to relieve forces due to the foreshortening of the compression side of the glass. As the bending is increased the buckles extend toward the center. The magnitude of the buckles are the greatest at the edge of the glass sheet, diminishing toward the center of the sheet.
The larger the buckles and the further the buckles extend into the glass sheet, the smaller the useable area of the glass. This useable area is referred to as the "quality area." The size of the quality area directly impacts the value of the glass. The larger the quality area, the larger the size of the final product--the curved glass--and the greater the profit. In optical quality glass or plastic, the tolerance for distortions in the surface of the glass is extremely low. Hence the size of the quality area for optical glass is relatively small when conventional glass bending techniques are used.
In addition to the limitations on the quality of the glass produced by conventional glass bending techniques, there are numerous process controls. The male and female molds require accurate alignment. This step can take up to two hours. Furthermore, the two rigid molds of the conventional glass bending apparatus require substantially perfect alignment of the heated sheet of glass between the two molds. If the glass sheet is not properly aligned, the peripheral edges of the sheet rest in a non-horizontal plane within the female mold so that when the female mold is moved toward the male mold, the male mold will first make contact with a portion of the glass that is off-set from the center of the glass sheet. Therefore, the glass sheet will have a bend that is off-center and, more than likely, the glass sheet will exhibit exaggerated buckling at least on one side of the plate. Moreover, the conventional glass bending apparatus requires the temperature of the heated sheet of glass to be controlled within about 5 degrees. Also, debris, such as broken pieces of glass, that may be inadvertently left in the molding apparatus could damage one or both of the molds during the bending process.
It will be understood from the foregoing that the conventional bending apparatus requires numerous control steps and inputs, because it is very sensitive to changes in the sheet size, the material properties of the sheet, the state of the sheet, i.e., its temperature and condition, alignment of the molds, and placement of the sheet within the assembly. At the same time, due to the variations in the glass sheet, the pressures exerted by the female and male molds are typically non-uniform and, consequently, result in less control in the bending process.
Consequently, there is a need for an improved glass bending apparatus and method that will reduce the size of the buckles of the glass sheet during bending, increase the size of the quality zone, provide a greater range of tolerance for the alignment of a heated sheet of glass within the molding apparatus, reduce the number of process steps and inputs required to control the process, and reduce the risk of damage to the molds, while at the same time providing more control and uniformity to the bending process.